Pulsed-cathode electron gun



1 1959 T. J. MARCHESE ETAL PULSED-CATHODE ELECTRON GUN Filed Feb; "7, 1956 E n ll; a 1 I m MR SRA RAM 0 T mW T. W a w r b b mwk M m 7R I 1 W I 6 ,H x q I I I PULSED-CATHODE ELECTRON GUN Theodore J. Marchese, Nutley, and Robert W. Wihnarth, Rutherford, N.J., assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Application February 7, 1956, Serial No. 564,023 11 Claims. (Cl. 315-14) This invention relates to electrode systems, commonly referred to as electron guns, for producing a concentrated beam of high-speed electrons for use in electron discharge devices, and, more particularly, to electron guns for pulsed electron discharge devices.

Many electron discharge devices, because of highpower requirements and various microwave radar applications, require pulsed operation. In one commonly used method of operating pulsed electron discharge devices having electron guns, one or more of the anodes are maintained at or near ground or chassis reference potential in order to shield the beam against disturbing electric fields, and the cathode and heater are electrically coupled and maintained at a high-voltage negative potential with respect to ground. Although for some applications the anode may be pulsed, in many instances'it is preferable to pulse the thermionic cathode.

Electron gun assemblies are well known. In one general type of gun structure used for producing a concentrated beam of high-speed electrons for use in cathoderay tubes, klystrons, traveling-wave tubes and similar devices, the electrode elements are arranged as a series of metal cylinders about a common axis. In operation of the electron gun, the electrons may start from a small spot of oxide coating on the end of usually an indirectly heated thermionic cathode and then pass through a control electrode consisting of a closed cylinder with a small central hole. This further restricts the dimensions of the beam, and a negative potential may be applied to this control electrode to control the beam intensity in a manner analogous to that in which the grid bias controls the plate current of a triode. may then be used to accelerate the electrons to a high velocity and focus them into a fine beam.

In pulsed-cathode operation, one of the problems encountered in the operation of a modulated-pulse hotcathode electrongun is-that of pulse distortion. Because the thermionic cathode and heater are close together and interconnected for efiicient heat transfer, the capacitance existing between the primary and secondary of the power transformer supplying the heater constitutes part of the load reactance and effectively lowers the load impedance. The capacitance between the cathode and anode also contributes to the capacitive reactance and results in a lowering of the load impedance. This, in turn, increases the size and cost of the modulator required and makes ,the attainment of a specific pulse shape more difiicult.

It is an object of the present invention to provide a novel construction of an electron gun having an indirectly heated thermionic cathode.

It is an additional object to provide a pulsed hotcathode electron discharge device having a substantially lower capacitance than those heretofore available.

It is a feature of this invention that an indirectly heated thermionic cathode for an electron gun is provided by heating the cathode to an electron-emitting temperature by bombarding it with electrons emitted by a heater electrically decoupled from the cathode.

One or two positive anodes V ment; and

a pulse train 2,912,616 Patented Nov. 10, 1959 It is a further feature that a low-capacitance pulsed electron discharge device is provided wherein only the cathode is pulsed.

The invention and other specific objects and features thereof will be understood more clearly and fully from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is an elevational View, partly in section, of the electron gun of this invention;

Fig. 2 is a diagrammatic representation of the electron gun in a pulsed-cathode circuit arrangement;

Fig. 3 is an alternative pulsed-cathode circuit arrange- Fig. 4 is a graphical representation of used for pulse modulating the cathode.

Referring to Fig. l, a portion of an electron discharge tube 1 is shown, particularly illustrating the electron gun 2 of this invention. The electron gun 2 comprises series of cylinders arranged about a common axis, the outermost cylinder consisting of a cup-shaped anode 3 having an aperture 4 therein for passage of the electron beam. In cooperative relation to this anode is disposed a dish-shaped cathode 5 having a coating 6 of an electronemitting material on its outer surface 7 facing the anode. A self-supporting heater 8 consisting of a tungsten wire coil 9 is placed in spaced relation to the cathode and electrically decoupled therefrom. Disposed about the heater is a metallic cylinder 10 for reflecting the electrons emitted by the heater. This cylinder 10 is supported by rods 11 and 12 which are attached to the base 13 of the electron discharge device. Electrodes 15 and 16 are used to supply current to the tungsten coil 9. Also connected to the cathode 5 and axially disposed thereabout is a control electrode 17 used for restricting the electron beam emitted to the anode 3. For some applications, the control electrode 17 may be electrically decoupled from the cathode 5 and maintained at a different potential therefrom. The anode 3 is electrically decoupled from the cathode 5 and control electrode 17 by a cylinder 18 made of an insulating material such as glass. In a similar manner, the cathode 5 is decoupled from the heater 8 and its associated elements by an insulating cylinder 19 of a similar material as that used for cylinder 18. The heater electrodes 15 and 16 are connected at terminals 20 and-21, and cathodic and anodic connections are made at the side of the device at points 22 and 23, respectively.

In operation, the secondary Winding 24 of a transformer 25, illustrated schematically in Fig. 2 but not shown in Fig. l, is connected to terminals 20 and 21 to supply power to the heater 8. The heater may be brought to a relatively high temperature because of the use .Of a tungsten coil 9. The cathode 5 is positively biased with respect to the heater, thereby increasing the flow to the cathode of electrons emitted by the heater. These electrons bombard the under surface of the cathode and serve to bring it to a temperature of thermionic emission. The heater 8 is sufficiently spaced apart from the cathode 5 so as to maintain as low a capacitance as possible between them consonant with efiicient electron bombardment of the cathode. The coating 6 on the outer surface 7 of the cathode 5 consists of any of suitable electron-emitting materials, such as various alkali and alkaline-earth oxides. Upon the cathode attaining a temperature of thermionic emission caused by bombardment by the emitted electrons from the heater, electrons will, in turn, be emitted by the cathode coating toward the anode. As mentioned, this flow of electrons may be controlled by suitably biasing the control electrode 17 and/or by biasing the anode 3. In essence, then, an electron gun has been provided wherein the cathode is electrically decoupled from the heater and functions as heater. While not preferable for the general purposes I of this invention, for specific applications the cathode may be heated to the emissive temperature by radiant heat from a heater 8. V V

In Fig.2 is illustrated the manner in which pulsing or modulation of the cathode takes place. An alternatingcurrent source 26 is'us'ed to supply power to the transformer 25. Thesecondary 24, of this transformer, as previously mentioned, is connected to terminals 29 and '21 of the heater 8. For many applications, high power .is required, and the transformer supplying the heater.

power istherefore usually massive in construction with a relatively high capacitance existing between the primary.

27 and secondary 24 of the transformer of an order of approximately 50 to 100 micromicrofarads' In the circnit arrangement illustrated in Fig. 2, the anode 3 and the heater 8 are both shown at ground potential. it is generally preferred'to maintain both the anode and the heater at the same reference potential for most applications'. However, it is an essential feature of this invention that the heater and the cathode are maintained at different potentials and are electrically decoupled from one another. Since in this circuit arrangement, the directcurrent power supply conducts the pulse-modulating current, it is desirable that the impedance of the power supply be at a minimum at the pulse generator frequencies.

In Fig. 3 is shown an alternative circuit arrangement for pulse-modulating the cathode. In this embodiment, terminals 20 and 21 of the heater 8 are connected to the secondary 24 of transformer 25. However, terminal 20 is additionally directly connected'to the direct-current power supply 29. In this manner, the direct-current power supply is associated directly with the heater circuit. Thereby, the flow of the pulse-modulating current through the direct-current power supply is eliminated. This may allow for a simplification of the direct-current power supply. I

In Fig. 4 is shown a graphical depiction of a pulse train, with the ordinate representing cathode-anode voltage, and the abscissa corresponding to time. While the pulse train has been illustrated with reference to the circuit of Fig. 2, it will be obvious that this analysis is equally applicable to Fig. 3. For most applications, arelatively low duty cycle is maintained, with the pulse being on approximately 0.1 percentof the time. The slope of the pulse, as well as its specific shape, is'a function of the loading'of the pulse generator which, in turn, is markedly aife'cted by the capacitance of the system. It is the marked reduction in capacitance accomplished by pulsing the cathode alone that is an important feature ofthis invention.

Referring to Figs. 2 and 4, it is seen that the pulse generator 28 is electrically coupled only to the cathode 5 and cannot, in its operation, aifect the heater 8, being electrically decoupled therefrom. Consequently, the relatively'high capacitance existing between the primary and secondary of thetransformer 25 can no longer affect the pulse shape. While a capacitance between the heater '8 and cathode 5 has now been introduced by electrically decoupling the heater and cathode from one another, this capacitance may be made relatively low by an appropriate spacing between the heater and cathode, and is ordinarily of an order of 57m micromi'crofarads. In operating the tube, the bombardment power supply 29 biases the cathode 5 rendering it positive with respect to the heater 8. Generally, it is preferable to use a directcurrent for the power supply 29 where it is desired to maintain a continuous positivebiason the cathode with resp'ect'to the heater. Thereby, electrons emitted by the theater readily flow to the cathode, bombarding it and bringing it to a temperature required for thermionic emission. However, inasmuch as the cathode is approximately one thousand volts positive'with respect to the anode, there is essentially no current iiow from the thermionically heated cathode to the anode. The cathode voltage during the ofi period of the duty cycle is shown as V a in Fig. 4. This corresponds to the bombardment power supply voltage. During the on period of the duty cycle, a negative pulse, V of an order of several thousand volts, frequently several hundred thousand volts and always of magnitude greater than the voltage supplied by the bombardment power supply,'is applied to the cathode by the pulse generator. As a consequence, the anode beco rncs'highly positive with respect to the cathode and a copious flow of electrons between the cathode and anode occurs. The net cathodeanode voltage is shown as V '-V' during this on period. 'While during this onfperiod of the duty cycle there is no bombardment of the cathode by electrons emitted by the heater because of the high negative bias on the cathode with respect to the heater, the duration of the on cycle, as mentioned, is very small compared to that of the off cycle; therefore,, only a slight temperature drop in the thermionically heated cathode occurs during the on period of the pulse. Because of the low capacitance of the cathode with respect to ground, the pulse shapes shown in Fig. 4 are relatively undistorted and show a very steep rise time. The main tenance of these pulse shapes free from distortion is of Computations fora pulsed-cathode circuit arrangement made in accordance with this invention-show that a 50 percent, reduction in capacitance can be readily achieved.

It is thus seen that by providing a pulsed hot-cathode electron tube wherein the cathode and heater are electrically decoupled from one ano ther, undistorted high-risetime pulses are readily attained. At the same time, by providing a cathode that is heated'by electron bombardment, the heater-cathode capacitance has been maintained at a low value, as might otherwise not have been feasible had the heater been placed in that close proximity to the cathode as would be required to heat the cathode by direct radiation of heat. In addition, troublesome problems of insulation between the heater and cathode have thereby also been eliminated.

While we have described above the principles of our invention in connection with specific apparatus and cir cuitry,fit is to be clearly understood that this description is madeonlyby way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

1. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material; and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, means heating said heater for'electron emission, means for maintaining a difference of electric potential between saidcathode and said heater for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emissive material to an electron emissivetemperature, means establishing a difference of potential between said'anode and said cathode to normally prevent electron flow from 'said heated electron emissive material toward said anode, and a pulse source coupled to said cathode to establish a potential difference between said anode-andsaid cathode to supply a copious flow of electrons from said heated electron emissive material toward said anode" during the duration of the 'pulseof-said pulse source only. v v

2. An electron "tube system comprising an electron gun 7 having an anode, a thermionic cathode disposed in co} operative relation with said anode, the surface of said emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, means heating said heater for electron emission, means for maintaining a difference of electric potential between said cathode and said heater for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emissive material to an electron emission temperature, means to bias said cathode with respect to said anode to normally prevent electron fiow from said heated cathode to said anode, and means to pulse said cathode to render said cathode negative with respect to said anode to provide a copious flow of electrons toward said anode during the duration of the pulse only.

3. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, means heating said heater for electron emission, means for maintaining a difference of electric potential between said cathode and said heater for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emission material to an electron emissive temperature, and a pulse source coupled to said cathode to pulse said cathode to supply a copious flow of electrons from said heated electron emissive mate rial toward said anode during the duration of the pulse of said pulse source only.

4. A system according to claim 3, where the pulse of said pulse source has a magnitude and polarity with respect to the magnitude and polarity of the potential difference between said heater and said cathode to interrupt the electron bombardment of said cathode during the duration of the pulse of said pulse source.

5. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said I cathode adjacent said anode being coated with an electron emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, means heating said heater for electron emission, means for maintaining a difference of electric potential between said cathode and said heater for accel eration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emissive material to an electron emission temperature, a point of reference potential, means connecting said anode to said reference potential and a pulse source coupled between said reference potential and said cathode for pulsing said cathode only to supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of said pulse only.

6. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, means heating said heater for electron emission, means for maintaining a difference of electric potential between said cathode and said heater for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emissive material to an electron emission temperature, ground potential, means connecting said anode to said ground potential to provide a potential on said anode with respect to the potential on said cathode to normally prevent electron flow from said heated electron emissive material toward said anode, and a pulse source coupled between said ground potential and said cathode to establish a potential difference between said anode and said cathode to supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of the pulse of said pulse source only.

7. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material and means for heating said electron emissive material to an electron emission temperature, means establishing a difference of potential between said anode and said cathode to-normally prevent electron flow from said heated electron emissive material toward said anode and a pulse source coupled to said cathode to establish a potential difference between said anode and said cathode to supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of the pulse of said pulse source only.

8. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material and means for heating said electron emissive material to an electron emission temperature, ground potential, means connecting said anode to said ground potential, means biasing said cathode with rmpect to said anode to normally prevent electron flow from said heated electron emissive material toward said anode, and a pulse source coupled between said cathode and said ground potential to establish a potential difference between said anode and said cathode to supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of the pulse of said pulse source only.

9. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, ground potential, a conductor connecting said heater to said ground potential, means coupling alternating current to said heater for heating thereof for electron emission, a positive direct current voltage source, means coupling said direct current voltage source between said cathode and said ground potential to render said cathode positive with respect to said heater for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat said electron emissive material to an electron emission temperature, a conductor connecting said anode to said ground potential to render said anode negative with respect to said cathode to normally prevent electron flow from said heated electron emissive material toward said anode, and a negative pulse source coupled to said cathode to render said cathode negative with respect to said anode to supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of the pulse of said pulse source only.

10. An electron tube system comprising an electron gun having an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material, and a heater in spaced relation with the other surface of said cathode and electrically insulated from said cathode, ground potential, means coupling said cathode to said ground potential, means coupling alternating current to said heater for heating thereof for electron emission, a negative direct current power supply, means coupling said direct current voltage source between said heater and said ground potential to render said heater negative with respect to said cathode for acceleration of the electrons emitted from said heater for electron bombardment of said other surface of said cathode to heat 7 "2 7 said electron Vemissiye material to an electron emission temperature, a conduetor connecting said anode to said ground-potential to render the potential difference between said anode and said cathode sufiicient to normally prevent electron flow from said heated electron emissive material toward said anode, and a negative pulse source coupled to said cathode to render said cathode negative with respect to said anode to supply a copious flow of electrons from said heated electron emissive material to- 1 ward said anode during the duration of the pulse of said pulse source only. V

11. An electron tube system comprising an electron gun having, an anode, a thermionic cathode disposed in cooperative relation with said anode, the surface of said cathode adjacent said anode being coated with an electron emissive material and means for heating said electron emissive material .to an electron emission temperature, a reference potential, means connecting said anode to said reference potential, means biasing said cathode with respect to said anode to normally prevent electron flow from said heated electron emissive material toward said anode, and a pulse source coupled between said cathode and said reference potential to establish a potential difference between said anode and'said cathodeto supply a copious flow of electrons from said heated electron emissive material toward said anode during the duration of the pulse of said pulse source only.

References Cited in the file of this patent UNITED'STATES PATENTS 1,839,863 Berthold Jan. 5, 1932 2,132,216 Nelson t, Oct. 4, 1938 2,239,416 Ehrenberg Apr. 22, 1941 2,410,822 Kenyon Nov. 12, 1946 2,509,053 Calbrick May 23, 1950 2.717.963 Brubaker Sept. 13, 1955 

